Police gave the toll as 183 dead and 624 injured in the attack on Tuesday.
The blasts hit trains and stations in the Matunga, Khar, Santa Cruz, Jogeshwari, Borivali and Bhayendar areas of Mumbai during rush hour.
Police said the blasts were in the first-class carriages of packed commuter trains.
"The blasts happened when the trains were most crowded," said DK Shankaran, chief secretary of the state of Maharashtra, of which Mumbai is the capital.
Police described the series of the apparently co-ordinated blasts as an "attack" and said they were caused by bombs.
All trains have been suspended and railway officials have appealed to the public to stay away from the city's stations.
Hundreds of passengers have
been injured in the explosions
Heavy monsoon rains hindered the rescue effort which was continuing late into the night.
Manmohan Singh, the Indian prime minister, called the explosions "a shameful attack" but urged people to remain calm.
"It is a sad day," VK Duggal, the Indian home minister, told reporters before a meeting with the prime minister. "Security has been definitely put on high alert."
Mumbai has been hit by a series of bomb blasts in recent years. More than 250 people died in explosions in the city in 1993, which authorities blamed on criminal gangs.
Police in New Delhi said they were prepared for violence in the Indian capital.
"We have mobilised our entire forces who are conducting checks in all areas such as buses, bus stops, train stations and religious institutions," Anil Shukla, deputy commissioner of police for South Delhi, told Reuters.